1. Field of the Invention
The present invention relates to a power supply apparatus for generating a plasma.
2. Description of Related Art
A power supply apparatus for generating a plasma, which may be referred to xe2x80x9ca power supply for plasmaxe2x80x9d simply and suitably, hereinafter, is known as a power supply used for the purpose of generating a stable plasma in a step of semiconductor manufacturing process, e.g., a step of, high-frequency sputtering, plasma CVD, plasma etching, plasma ashing or the like.
A conventional power supply for plasma has an amplification system which comprises a multi-stage linear amplifier, that is, amplifiers connected in a multi-stage, for amplifying minute oscillations generated by a built-in quartz oscillator, up to the final output in order. The amplification system used in the power supply is called as the linear amplification system which has a relatively low efficiency of about 50%.
However, use of such a conventional amplifier with a low amplification efficiency cannot satisfy the requirement of a large output of power supply for plasma by the market because of making the volume and the power loss of the power supply apparatus very large. For the reason, recently, the so-called switching mode amplifier which has an amplification system with an efficiency higher than that of the conventional one, i.e., more than 80%, is mainly used. The switching mode amplifier includes two amplification systems, that is, the so-called Class-D and Class-E. For the power supply for plasma, the amplification system of Class-E is generally used.
FIG. 17 shows a circuit construction of a general amplification system of Class-E. In the system of Class-E, a switching element QS and a choke coil RFC are connected in series with respect to an input voltage VDD. A sine wave output is obtained from the connection between the switching element QS and the choke coil RFC through a series resonant circuit which comprises a resonant capacitor CO and a resonant reactor LO.
In the system of Class-E shown in FIG. 17, the peak value of voltage applied to the switching element QS becomes large to exceed three times the voltage VDD of the power supply even in a steady state of the load because the switching element QS is connected to the power supply VDD through the choke coil RFC. Further, in a plasma generating apparatus in which there are steep fluctuations in load, the load may change from open circuit to short circuit and vice versa in a short time, in a transient phenomenon, so that an excess voltage may be often applied to the switching element QS depending on the change of states. There is a problem that the switching element QS may be destroyed in a moment when the voltage applied to the switching element QS exceeds the allowable voltage once.
An object of the invention is to realize a power supply apparatus for generating a plasma which has a high efficiency and can protect the switching element from steep transient fluctuations peculiar to plasma load.
In accordance with an aspect of the invention, the power supply apparatus for generating a plasma for supplying a high-frequency power to a plasma generating device which is a load, comprises:
a DC power supply (for example, a variable DC power 10 in FIG. 1);
a power conversion circuit (for example, amplifier circuits 20 in FIG. 1) which comprises an amplifier circuit of Class-D comprising a plurality of switching elements and which converts a DC power output of the DC power supply to a high-frequency power to output; and
a load impedance conversion circuit (for example, a load impedance conversion circuit 30 in FIG. 1) which converts a load impedance to a predetermined delayed load,
wherein the power supply apparatus is adapted to supply the high-frequency power output from the power conversion circuit to a plasma generating device through the load impedance conversion circuit.
According to the power supply apparatus of the invention, because the power conversion circuit comprises an amplifier circuit of Class-D comprising a plurality of switching elements and which converts a DC power output of the DC power supply to a high-frequency power to output, it is possible to suppress the voltage applied to each switching element up to the voltage maximum supplied by the DC power supply. Accordingly, even if the power supply apparatus is used for a plasma generating apparatus in which there are steep fluctuations in load in a transient phenomenon, it is possible to prevent destruction of the switching element in the power supply apparatus.
Further, because the load impedance is converted to a predetermined delayed load with respect to the power conversion circuit by the load impedance conversion circuit, the load current is also delayed in phase. As a result, it is possible to reduce loss in the circuit which may be occurred by switching operations and to realize a power supply apparatus for generating a plasma with a high efficiency.
To be concrete, the load current is delayed in phase with respect to the power conversion circuit so that the direction of the load current is changed after an elapse of a predetermined time after the semiconductor switches turn on, for example, a time longer than one corresponding to the recovery time for the parasite diodes of the semiconductor switches. Thereby, it is possible to prevent short-circuit current flowing through the amplifier circuit and therefore to reduce the loss which may be occurred by switching operations.
Preferably, the power conversion circuit comprises a full bridge inverter circuit in which the plurality of switching elements are bridge-connected.
The above-described predetermined delayed load preferably makes a phase of load current delay so that a direction of the load current is reversed after an elapse of a predetermined time since one of the switching elements turns on.
The predetermined time may be longer than a time corresponding to a recovery time for parasite diodes of the one of the switching elements.
The load impedance conversion circuit may comprise a reactance Xs connected to one of input terminals thereof in series and a reactance Xp connected to the input terminals in parallel, each of the reactance Xs and Xp comprising a coil or a capacitor, to determine the phase of load current delayed.
Preferably, the power supply apparatus further comprises: a composition circuit for composing outputs of the plurality of amplifier circuits which are supplied through the load impedance conversion circuit, to generate a high-frequency power with a high voltage; a filter circuit for cutting off harmonic components from an output of the composition circuit; a power sensor for detecting an output power of the filter circuit; and a control circuit for controlling a power conversion operation of the DC power supply on the basis of a detected value of the power sensor.
Preferably, each of the amplifier circuits of Class-D comprises: first, second, third and fourth semiconductor switches which are bridge-connected; and a transformer, a terminal of a primary winding of the transformer being connected to a connecting point of the first and second semiconductor switches and the other terminal of the primary winding being connected to a connecting point of the third and fourth semiconductor switches.
The first, second, third and fourth semiconductor switches have first, second, third and fourth parasite diodes, respectively; and the predetermined delayed load allows the amplifier circuit to transfer from the first and fourth parasite diodes being ON to the first and fourth semiconductor switches being ON, so that no excess current nor excess voltage are generated in the second and third semiconductor switches and in the second and third parasite diodes, and to transfer from the second and third parasite diodes being ON to the second and third semiconductor switches being ON, so that no excess current nor excess voltage are generated in the first and fourth semiconductor switches and in the first and fourth parasite diodes.